1. Field of the Invention
The present invention is directed in general to biometric identity verifier devices, and more specifically to optical fingerprint identity verifier hardware systems.
2. Description of Related Art
Existing optical fingerprint sensors generally use a right angle prism to capture the fingerprint image. The characteristic optical path in such sensors is illustrated in FIG. 1. Collimated light rays from a light source pass through one side of the prism. These light rays then travels to the top of the prism where a finger is placed. If a light ray impinges on a location where the finger is not in direct contact with the top of the prism, the light ray undergoes total internal reflection, and emerges through the other side of the prism where it is captured by an imaging sensor that is oriented perpendicular to the light ray. Alternatively, if the light ray impinges on a location where the finger is in direct contact with the top of the prism, the light ray does not undergo total internal reflection but is instead absorbed by the skin. Thus, the optical fingerprint sensor captures a pattern of light and dark bands representing the fingerprint.
As a consequence of the characteristic optical path described above, existing optical fingerprint sensors have the following drawbacks: (1) the sensor module has a large volume thereby limiting the range of commercial applications, (2) the non-planar orientation of the light source and the imaging sensor increases the cost to manufacture the sensor module and also limits commercial applications, and (3) the fingerprint image captured by the image sensor is compressed along one direction necessitating software correction of the fingerprint image which increases the cost to manufacture the sensor module and further limits commercial applications.
U.S. Pat. Nos. 5,892,599 and 5,629,764 added a holographic phase grating to the prism so that the image capture plane is normal to the optical axis of the imaging lens in order to remove the image distortion. U.S. Pat. No. 6,240,200 added a series of prisms as an alternative means to remove the image distortion. The problem with these approaches is that they necessitate a bulky and expensive sensor module.
U.S. Pat. No. 5,796,858 used a grating to replace the prism. The problem with this approach is that the fingerprint image quality suffers due to the optical imperfections of the grating.
Accordingly, there is a need for an optical fingerprint sensor that reduces the size and cost of the sensor module while ensuring a high quality of the fingerprint image.